With Facebook evolving into a major rival, Google is meeting the challenge by building its own social network centered on its lucrative search business.

On Wednesday, Google introduced the "+1" (plus-1) service that would let people who connect to friends using their Google Profile to share their recommendations for websites and online advertisers on search results. Google hopes to eventually expand the feature to include a person's friends on other social networks.

The service has the potential of changing the way people use Google's search engine. Today, search queries are answered through machine algorithms. In the future, the website recommendations of people and their friends could also play a role in how results are displayed.

"What I like about this is it give you the best of both worlds," Allen Weiner, an analyst for Gartner, said in an interview. "It gives you a machine answer and it gives you a human curated answer by adding recommendations from other people."

The way that will be done is by people clicking on the "+1" that will start appearing on websites and online advertising. People in a person's social network within the Google community will then see the symbol on results fed to a related search query. The symbol could eventually be useful to people searching for products and services, since experts agree that word-of-mouth is the most powerful marketing tool.

Facebook has a similar feature called the "like" button. People on the social network click the button to show approval of a picture, link, or other content on Facebook, the world's largest social network with roughly a half billion users. Facebook poses a threat to Google's business, because people are using the network as a way to find products and services through recommendations of others, rather than search engines. "The biggest threat that search faces today is social networks," Weiner said.

Google's +1 would take on the challenge by first leveraging those people who have a Google Profile. Similar to a Facebook profile, the page provides general information about a person, such as occupation, education, the city where they live, and more. On Google, the profile can be connected to contacts a person has on other services provided by the company, such as Picasa photo-sharing, YouTube, or Gmail.

People who take advantage of the +1 feature will have their recommendations appear on the search results delivered to people on their contact lists. Rob Spiro, product manager for +1, said on the Google blog that a person's contacts on other social networks, such as Twitter, could also be tapped sometime in the future. There was not mention of Facebook in the blog.

Connecting +1 to social networks outside of Google services will be critical to building a large enough community to make the feature useful to people. "There's really not enough people in the Google community to make it work," Weiner said.

Google is rolling out the feature starting in English on Google.com. People who want to get started immediately can opt-in through the company's experimental search site. Advertisers will be able to participate through Google's AdWords service.

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